Wait til you see the number of children who are not fathered by the mother's husband. This is one of the concerns genealogists face with some of the family DNA attempts being made.
There are reports of “problems” in that direction.
I am dealing with that exact subject right now. I just got notice that an older man matches our yDNA 25/25, but he is not of the same surname. However, his mother is of the same surname. I am thinking he may be an uncle's child. He doesn't realize the problem, as he doesn't understand the ydna should go son to father to grandfather etc... Now the dilemma is should we tell him? He's about 80 years old.